Investigations of conductivity in FEP-based radiation-grafted alkaline anion-exchange membranes

A series of radiation-grafted alkaline anion-exchange membranes (AAEMs) with degrees of grafting (by mass) in the range 20-26% were produced and characterised. Conductivities (from impedance spectroscopy) up to 0.023±0.001 S cm−1 at 50±1 °C were demonstrated, which are between 20-50% of values for the commercial acid-form membrane Nafion®-115 [N115; σ=0.045±0.004 (65 °C)-0.104±0.008 (50±1 °C) S cm−1, depending on the level of hydration (10 and 23 water molecules per sulfonate group, respectively)]. This significant result clearly indicates the suitability of these AAEMs for application in fuel cells at temperatures below the widely accepted alkaline membrane thermal stability limit of 60 °C. Statistical analysis of the results of ion-exchange capacity and conductivity studies on the AAEMs indicates the presence of heterogeneity in the grafting levels within the AAEMs and suggests that more synthetic optimization work is required. The empirical activation energies (Arrehnius behaviour assumed) for the hydroxide ion migration in the AAEMs (12.6±0.6 kJ mol−1 at the 95% confidence level) are twice the value for proton migration found for fully hydrated Nafion®-115 (6.0±0.8 kJ mol−1) and show that hydroxide ion mobility is more strongly temperature-dependent.